To prevent the dire consequences of uncontrolled activation of tyrosine kinase receptors, such as the epidermal growth factor receptor (EGFR), both extracellular and intracellular controlling mechanisms have been devised during evolution. One such extracellular mechanism of EGFR regulation is provided by decorin, a leucine-rich proteoglycan that binds to and downregulates EGFR activity both in vitro and in vivo. We hypothesized that other soluble forms of proteins harboring leucine-rich repeats could similarly affect the EGFR signaling pathway. Thus, we focused our attention on LRIG1, a transmembrane protein containing fifteen leucine-rich repeats and three Ig-like repeats in its ectodomain. We generated a soluble ectodomain of LRIG1 containing only the leucine-rich repeats and flanking Cys-rich caps. We discovered that nanomolar concentrations of LRIG1 ectodomain inhibit both basal and ligand-dependent EGFR activation and Erk1/2 signaling in a dose-dependent fashion. This, in turn, causes growth inhibition of EGFR-expressing carcinoma cells but not of cells lacking expression of the receptor. Furthermore, we provide genetic evidence for EGFR requirement in the LRIG1-mediated function, and demonstrate the existence of high-affinity (Kd=10 nM) binding sites on the A431 cells, the vast majority (up to 75%) of which can be competitively displaced by EGF. These novel results suggest that the soluble ectodomain of LRIG1, which could conceivably be released at sites of tissue remodeling and tumor invasion, might act as negative regulator of EGFR activity. The central hypothesis of this new grant application is that release of soluble LRIG1 ectodomain from the cell surface around or within carcinomas could represent a biological mechanism to counteract the invading tumor cells, thereby functioning as a natural EGFR antagonist. Specifically, we plan to: [1] Decipher the mechanism of action of LRIG1 ectodomain in suppressing EGFR activity, [2] Determine the mechanism of LRIG1-evoked signaling via the EGFR and its ability to inhibit cell growth , and [3] Investigate the in vivo function of LRIG1 ectodomain as an anti-oncogenic factor. These concerted research lines should firmly establish the functional roles of LRIG1 in tumorigenicity and shed light on its mechanism of action. The expected results could open novel perspectives for basic cancer research, and could lead to future approaches of cancer treatment by using a natural inhibitor of tumor cell growth.